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Introduction to space weather
Advances in Space Research 35 (2005) 855–865 www.elsevier.com/locate/asr
Introduction to space weather E. Echer *, W.D. Gonzalez, F.L. Guarnieri, A. Dal Lago, L.E.A. Vieira Instituto Nacional de Pesquisas Espaciais (INPE), Minsterio da ciencia e Technologia, Av.dos Astronautas, P.O. Box 515, Sa˜o Jose´ dos Campos 12245-970, SP, Brazil Received 1 June 2004; received in revised form 27 January 2005; accepted 22 February 2005
Abstract The solar and interplanetary origin of space weather disturbances, as well as the related magnetospheric dynamics, will be presented. Besides the involved phenomenology in solar–terrestrial physics, some of the main effects of space weather variability concerning mankind in space and at the earthÕs surface will also be discussed. The November 2003 event is shown as an example of the solar, interplanetary and magnetospheric aspects of a space weather storm. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Space weather; Solar effects; Magnetospheric dynamics; Solar–terrestrial physics
1. Introduction In the last decades many technological advances occurred and mankind has become more dependent on the space systems and satellite-based services, and from other ground-based technologies, which are influenced by Sun–Earth interaction phenomena. The region where the electromagnetic Sun–Earth interactions are more important is called the Geospace (Hargreaves, 1992) and it encompasses the EarthÕs upper atmosphere (neutral and ionized), the magnetosphere and the local solar wind medium. Both academic and practical interests in the study of this environment led to the creation in recent years of a new field, the Space Weather, which was formerly known as solar–terrestrial physics. The objective of space weather research is to understand in theory and practice the, stormy and hostile Geospace environment, in order to make possible procedures that turn it safe for human technological activities and for the human presence in space (Hargreaves, 1992; Baker, 2000; Siscoe, 2000; Cole, 2003). *
Corresponding author. Fax: +55 12 3945 6810. E-mail address: [email protected] (E. Echer).
This environment is a complex system with three parts: the Sun and its atmosphere as the origin of the energy, the interplanetary space as the propagation medium and the EarthÕs magnetosphere and upper atmosphere as the forced system where the energy originated at the Sun and propagated through the interplanetary space is deposited. The occurrence of solar activity manifestations propagating through the interplanetary space and interacting with the terrestrial magnetosphere origins phenomena known for centuries, such as auroras (Chamberlain, 1961; Akasofu, 1964) and geomagnetic field disturbances – the geomagnetic storms (Chapman and Ferraro, 1931). Geomagnetic storms are usually defined by the geomagnetic field horizontal component variations, but they actually are disturbances in the plasma populations present in the entire magnetosphere. Nowadays, it is well known that the prime cause of geomagnetic storms is the presence of a southward interplanetary magnetic field structure in the solar wind. This magnetic field orientation makes possible the energy to be transferred to the EarthÕs magnetosphere through reconnection mechanism (Dungey, 1961; Gonzalez et al., 1994, 1999; Kamide et al., 1998). Although geomagnetic storms have
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.02.098
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been studied for several decades, their solar and interplanetary origins have become more clear only with more recent observations, especially in the last 10 years, when a continuous monitoring of the solar activity and interplanetary propagation conditions has been done. Nevertheless, the solar-interplanetary space–magnetospheric–atmospheric chain is very complex and there are many points to be further studied and better understood. The aim of this short review is to give some basic information on this environment, showing an example of a space weather storm during the beginning of November 2003. A brief summary of space weather effects on technology systems is also presented.
2. Solar activity The energy driving the Geospace processes has its fundamental origin in the SunÕs core. Nuclear fusion inside the SunÕs core generates radiative energy that propagates outward, and subsequently is transformed in convective motion energy in sub-surface layers. The dynamo action produces intense magnetic fields in the solar atmosphere, which are transported outward to the interplanetary space with the plasma in the solar corona, constituting the solar wind (Parker, 1958; Neugebauer and Snyder, 1966; Hundhausen, 1972; Kivelson and Russell, 1995). The magnetized solar wind flows continuously by the interplanetary medium and interacts with planetary magnetic fields, delimiting their magnetospheres. Besides the presence of this background and continuous solar wind, the interplanetary space is permeated in solar activity transients, e.g., the remnants of solar flares and coronal mass ejections (CMEs). The easiest observable characteristic of the solar variability is the number of sunspots in the visible solar hemisphere. The records of the observed sunspot number show an average regular cycle of solar activity near 11 years (Eddy, 1976). Fig. 1 shows the annual average
Fig. 1. Wolf and group sunspot number annual averages 1610–2000.
Wolf sunspot number (RZ), which is available since 1700, and the group sunspot number (RG) which is available since 1610. It is easily seen the 11-year oscillation, as well as the long-term variations, with sunspot maxima alternatively higher and lower. Years 1800– 1830 are particularly low solar activity period, being known as the Dalton minimum. The period of 1645– 1715 had the lowest sunspot activity ever observed, being called Maunder minimum (Eddy, 1976; Hoyt and Schatten, 1998). Solar transient events occur sporadically, but its frequency is higher around sunspot maximum. The sunspot number is considered then a general indicator of the solar magnetic activity. Nevertheless, the RZ is known to be less accurate before 1850. The group sunspot number RG was idealized to be a better descriptor of the solar activity, being available since 1610 (Hoyt and Schatten, 1998). Both RZ and RG data are available through Internet, from the National Geophysical Data Center, Boulder, Colorado (http://www.ngdc.noaa.gov/). Sunspots are solar disk regions that are darker than the vicinity areas and contain strong and transient magnetic fields. They are formed and dissipated over periods of days to weeks, more rarely persisting for a few solar rotations. They occur when strong magnetic fields emerge through the solar surface, partially blocking the plasma convection from the bottom and allowing the sunspot area to be cooler than the photosphere, around 4200 C against 6000 C of solar background. This difference in temperature is the reason why this area appears as a dark spot on the solar surface. The darkest area at the center of a sunspot, called umbra, is the region where the magnetic field strengths are the highest. The less dark, striated area around the umbra is called penumbra. Sunspots co rotate with the solar surface, taking about 27 days to make a complete rotation as seen from Earth. Sunspots near the SunÕs equator rotate at a faster rate than those near the solar poles. Groups of sunspots, especially those with complex magnetic field configurations, are often the sites of flares (Svestka, 1976; Sturrock, 1980). Fig. 2 shows an example of sunspots seen on solar disk through the SOHO Michelson Doppler Imager ˚ (http:// (MDI) instrument that operates at 6767 A umbra.nascom.nasa.gov/images/latest_mdi_igram.gif). This image is for the November 03, 2003 event. The October–November 2003 months were a period of intense solar activity. Giant sunspot regions 486 and 488 were seen near the solar limb. The solar magnetic field extends through solar atmosphere to the interplanetary space. The coronal gas is ionized and it is an excellent electrical conductor, despite its low density. Thus this gas is free to move parallel to magnetic field lines, but not perpendicularly. The solar corona structure is complex, but it is basically constituted of two regions characterized by open and closed
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˚. Fig. 2. Solar disk observed by SOHO MDI instrument at 6767 A Sunspots are seen as dark regions on the disk. Daily image for November 03, 2003.
magnetic field lines, shown schematically in Fig. 3. In space physics, open magnetic field lines are the ones that stretch themselves to long distances in the interplanetary space, reconnecting at long distances in the heliosphere. Open field lines occur in coronal holes, which emit the high speed solar wind streams. A closed magnetic field line is anchored in the photosphere in two points, being stretched in the corona as a loop. Low speed solar wind streams are originated in the helmet streamer and heliospheric current sheet. The helmet streamer is located, during solar minimum, close to the solar equatorial plane, while during solar maximum it is found over a large latitudinal range (Schatten, 1971).
Fig. 3. Simplified scheme of the solar corona magnetic field topology – open (coronal holes) and closed (helmet streamer) magnetic field lines are drawn.
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Large disturbances in the space weather, such as intense geomagnetic storms, shock waves and energetic particle events are mostly associated with two solar activity transient phenomena: solar flares and coronal mass ejections (CMEs). These two events seem to be part of a single phenomenon, a solar magnetic eruption. Nowadays it seems that neither one is the cause of the other (Gosling, 1993). Solar flares are intense, temporary releases of energy that were first observed by Carrington in 1859. sunspot. The primary energy source for flares appears to be the tearing and reconnection of strong magnetic fields. Flares radiate throughout the electromagnetic spectrum, from gamma rays and X-rays, through visible light out to kilometric radio waves. Flares are important to space weather mainly because they appear in connection to some CMEs and also because they have an important role in particle acceleration. The enhanced X-ray and extreme ultraviolet (EUV) solar radiation during a flare causes a dramatic increase in ionospheric ionization, with several consequences for radio-propagation and telecommunication systems (Svestka, 1976; Sturrock, 1980). Solar flares are classified according to its X-ray emis˚ in emission classes B (with sion in the band 1–8 A 6 2 peak < 10 W m ), C (peak between 10 6 and 10 5 W m 2), M (with peak between 10 5 and 10 4 Wm 2) and X (with peak > 10 4 Wm 2). A number is also indicated after the letter which gives the intensity of emission, each category having nine subdivisions, e.g., C1–C9, C9 equivalent to a peak emission of 9 · 10 5 W m 2. Events of the X type are big events that can cause planetary radio blackouts and long lasting radiation storms. Events of the M type are of medium size, which can cause minor radiation storms and brief radio-blackouts, mainly in the polar regions. Events of C and B types have very few effects on Earth. The outer solar atmosphere, the corona, is permeated by strong magnetic fields (Fig. 3). In the closed field regions, often above sunspot groups, the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas and magnetic fields called coronal mass ejections. A large CME can contain 1016 g of matter that can be accelerated to several thousands of km per seconds. CMEs usually travel at speeds between 500–1500 km/s and take 2 or 4 days to cross the 150 · 106 km that separates the Sun from the Earth. Less frequent, very fast CMEs can reach earth in one day or less. These very fast CMEs are often associated with flares, while the slow ones not. CMEs are monitored using coronagraphs, which produce artificial eclipses on the Sun by placing an Ôocculting diskÕ over the image of the Sun. CMEs directed along the Sun–Earth line can be detected as ‘‘halos’’ around the occulting disk in a white-light coronagraph. A CME is a full-halo when it extends 360 around the Sun and a partial halo
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Fig. 4. SOHO observations of 04 November 2003 event – solar flare (EIT) LASCO C2 (CME).
if the apparent width is greater than 120 and if it appears in projection above at least one of the poles; this latter criterion is an attempt to exclude limb events that just happen to be large but do not have a component along the radial Sun–Earth line. Limb CMEs occur near solar limb and are not aimed to the Earth (Gosling, 1997). Fig. 4 shows an example of a solar flare and a CME occurred on November 04, 2003. This flare is considered the biggest ever recorded in X-rays measurements by GOES, classified as of X28 type flare. The image is from SOHO Extreme ultraviolet Imaging Telescope (EIT) Fe ˚ line. The associated CME is shown in Fig. XII 195 A 4(b), as seen from SOHO Large Angle and Spectrometric Coronagraph (LASCO) C2 in white light. Both events were associated with the big sunspot groups seen in Fig. 2. The CME in Fig. 4 is a halo CME.
3. Interplanetary propagation The region between the Sun and the planets has been named the interplanetary medium. Although in the past it was considered a perfect vacuum, since the beginning of the 1960s it is known to be a turbulent region dominated by the solar wind, which flows at velocities in the range of 250–1000 km/s. Other characteristics of the solar wind, such as density, composition, and magnetic field strength, among others, vary with changing conditions on the Sun. The effect of the solar wind can be seen in the tail of comets, part of which always points away from the Sun. Near Earth, the solar wind is basically a proton–electron gas that flows from the sun (and outward to the entire solar system), with a velocity typically around 400–500 km/s, density around 5 cm 3 and interplanetary magnetic field strength of 5 nT. It is the solar atmospheric gas escaping as a result of the high temper-
ature corona expansion. Solar wind expansion is supersonic, which means that it propagates with higher velocity than the characteristic velocity of the medium – the magnetosonic speed (Hundhausen, 1972). Since the solar wind is high conductive plasma, the solar magnetic field is frozen in it and magnetic field lines are dragged by the solar plasma flow. As the Sun rotates, the solar magnetic field is deformed in a spiral form – the Parker spiral – whose angle is around 45 from the Sun–Earth line near 1 AU (1 AU = average Sun–Earth distance, 150 · 106 km). The solar wind flow deforms magnetic obstacles, such as planets and their satellites, but the resulting structure is dependent on their magnetic fields and atmospheres. Considering the Earth, its dipolar magnetic pattern field is deformed, compressed in the sunward direction and stretched out in the anti-solar direction, forming a long magnetotail. This interaction creates the magnetosphere as a complex magnetic cavity around Earth (Russell, 1972; Kivelson and Russell, 1995). The interplanetary counterparts of coronal mass ejections (ICMES) at the Sun have been identified since the early years of solar wind observations. A sub-set of ICMEs are the magnetic clouds (MC), which seem to constitute around 1/3 of all the ICMES (Gosling, 1997). These structures are identified, near 1 AU, by their high magnetic field intensity, low proton temperature, low Beta (ratio between thermal and magnetic pressure), and a smooth and large scale rotation in one of the magnetic field components. ICMEs present dimensions around 0.2–0.3 AU and cross the spacecrafts or Earth in 24 h (Burlaga et al., 1981). Current models of these magnetic clouds consider them as giant magnetic flux ropes with field aligned currents. Other ICMEs are believed to be ‘‘complex ejecta’’, with a disordered magnetic field (Burlaga et al., 2001). Presently it is unknown whether all ICMEs are flux ropes or not.
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It has been suggested that they all could be magnetic clouds, that are observed as complex structure due to solar wind interactions (e.g., with co-rotating streams or other ICMEs), or due to the spacecraft measuring its parameters far from the ICME central axis. On the other hand, one can not discard the possibility of different magnetic field topologies be expelled from the Sun. Plasma and magnetic field (5 min resolution observations) observed by instruments on board of the Advanced Composition Explorer (ACE) spacecraft are shown in Fig. 5. Panels are, from top to bottom, proton temperature Tp (in 105 K), solar wind speed Vsw (in km/s), solar wind proton density Np (in cm 3), magnetic field strength B (nT), magnetic field BX, BY and BZ components in GSE (Geocentric Solar–Ecliptic coordinates) and the plasma b parameter (b = pk/pB, kNpTp/(B2/l0)) Two shock waves are indicated by the continuous lines. A southward intense structure is indicated with Bs and gray color. Collisionless shock waves are disturbances propagating in the solar wind with relative velocity (to solar wind) higher than the characteristic velocity, and are usually driven by the interplanetary remnants of CMEs–ICMEs. They are seen in the interplanetary data as an abrupt increase in plasma and magnetic field parameters. In the present example, it was not possible to find the signals of an ICME, which means that it probably missed the Earth. The shock was observed be-
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cause it has a larger extent in area than the driving interplanetary structures (Schwenn, 1986). The first shock was driven by the CME associated with the X8 flare on November 02, 2003, and passed by ACE spacecraft at 0600 UT on November, 04, 2003, with Vsw increasing from 500 to 800 km/s. A turbulent BZ variation is present, reaching 20 nT during a few hours, then returning to north conditions and to 0 values after. Solar wind speed gradually declined to 500 km/s on November 06, until a second CME shock arrives. This was driven by the CME associated with the X28 flare on November 04 (Fig. 4) and that arrived at ACE on 06 November with Vsw reaching 625 km/s. Large solar proton events contaminated the plasma instrument onboard ACE, making data unreliable during November 02–03. The BS structure shown in this Figure was responsible for a geomagnetic storm that followed it on November 04. The basic nature of geomagnetic field-solar wind interactions was first showed by Chapman and Ferraro (1931). This interaction is based on two theoretical principles: Plasma and magnetic fields behaves approximately as if they were frozen in to one another. This is a consequence of the FaradayÕs law, because in electrically conductive plasma, the electric field in the rest frame should be close to zero, or very large electrical currents will be induced. Magnetic field lines are then
Fig. 5. Interplanetary plasma and magnetic field parameters observed by ACE spacecraft during the November 3–6, 2003 period. Two interplanetary shocks are marked by the continuous vertical lines.
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transported by plasmas and they are warped and twisted as the flux moves. Example: the IMF is twisted in a large spiral structure due to solar rotation. Effect of magnetic field on the plasma: it comes from the Lorentz force, experimented by a charge q moving with velocity in a magnetic field. Summing up on all charges in a given region, the net force generally is opposite to the inclination and twist of the magnetic field, or to its compression, in the frozen flux. There are two components of this force: (a) The magnetic field does an effective pressure on the plasma proportional to the square magnetic field magnitude. This force opposes to compression/rarefactions in the magnetic field; (b) The warped/twisted magnetic field lines do stress forces over the plasma. These forces oppose to the warping and twisting in magnetic field lines. As the solar wind plasma is frozen in the interplanetary magnetic field, and the terrestrial magnetosphere plasma is frozen in the geomagnetic field, the two plasma populations do not mix. The magnetosphere is a ÔclosedÕ system when the IMF BZ is northward, but it becomes an open system when BZ is southward, due to mass–energy transfer through reconnection mechanism (Dungey, 1961). The general evidence is that the dawndusk solar wind electric field drives the magnetospheric convection. These electric fields are caused by a combination of solar wind velocity and to the north–south interplanetary magnetic field. From these two parameters, the IMF north–south component is the most important because of its largest variability (Gonzalez et al., 1994, 1999).
4. Magnetospheric impacts Around the middle of the XIX century, the disturbances that showed a large decrease in the horizontal geomagnetic field component in low latitudes were denominated magnetic storms (Chapman and Bartels, 1940; Kamide et al., 1998). The characteristic signal of a magnetic storm is the horizontal (H) component depression of the geomagnetic field during several hours. This depression is caused by the intensification of the ring current circulating Earth in the westward direction and it is monitored by the Dst index. A magnetic storm sometimes presents a sudden increase in the H component of the geomagnetic field (storm sudden commencement) followed by an arbitrary period during which the enhanced field do not change substantially (initial phase). This is followed by a reduction in the H component during some hours in the main phase and ending with a slow recovery phase, during tens of hours. The initial phase and the sudden impulse are not always
present in magnetic storms, which are characterized essentially by the presence of the main and recovery phase (Gonzalez et al., 1994; Kamide et al., 1998). Fig. 6 shows an example of the Dst index variation during a magnetic storm, for the November 3–6, 2003 period. The storm presents all the characteristics of a typical storm during solar maximum conditions, with an initial phase and sudden impulse (due to the first interplanetary shock showed in Fig. 5), followed by a main phase, which was caused by the sheath fluctuating field after this shock. A large sudden impulse, 30 nT, is seen due to the strong magnetospheric compression by the shock. The main phase is very fast, characteristic of BS sheath caused storms. This storm reached a minimum of 90 nT, being a moderate storm, not reaching the intense storm criteria, Dst < 100 nT (Gonzalez et al., 1994, 1999). The recovery phase occurred during November 5 and 6, being terminated by the second sudden impulse due to the arrival of the second shock wave. The Dst index is based on the hourly average variations of the H component recorded at four low-latitudes observatories, after subtracting the average solar quiet variation and the permanent magnetic field from the disturbed one. It is available since 1957. The depression in the H component of the geomagnetic field is caused by an enhancement in the ring current, which is constituted primarily by energetic ions that flow in the westward direction in the region of 4–6 terrestrial radii, during the growth of the storm main phase. The recovery phase is characterized by a decay of the ring current due to a combination of several different energetic particle losses. The processes mostly considered for the decay of the ring current are: charge exchange, Coulomb collisions, wave-particle interactions, outflow of ring current ions through the dayside magnetopause. The Dst index is thus a measure of the kinetic energy carried by the particles in the ring current. Ideally the Dst index should be a measure of the symmetric ring current strength, but
Fig. 6. Geomagnetic storm of 4 November 2003. The storm phases are marked.
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the H component measured at any station contains effects of many other current systems, such as the magnetopause current, partial ring current and tail current, among others (Akasofu, 1983; Kivelson and Russell, 1995). Fig. 7 shows other parameters measured near Earth: 5-min averaged integral electron flux (electrons cm 2 s 1 sr 1) with energies greater man 2 MeV observed at GOES-12 geostationary satellite; 5-min averaged X-ray flux (Wm 2) measured by GOES-12 in ˚ band; the 3-h Kp index. 0.5–4 and in 1–8 A The 3-h Kp index represents the intensity of the planetary magnetic activity as seen at sub-auroral latitudes. The K index, derived for each of the contributing midlatitude observatories, reflects the maximum range of a component of the field over the 3-h time interval at each station. The Kp index is the average of the K values from all contributing observatories. A conversion scale transforms the quasi-logarithm Kp to a linear index named ap. This index is available since 1932 (Rostoker, 1972; Menvielle and Berthelier, 1991). Other index widely used is the auroral eletrojet (AE) index, which is idealized to obtain a measure of the strength of the auroral eletrojets. It is calculated as the difference between the upper (AU index) and lower (AL index) envelopes of the magnetic field H-component disturbances in the auroral zone (Rostoker, 1972). The Kp/ap index is, then, a mid-latitude index that is sensitive to both auroral phenomena and to the equatorial ring current index. The auroral phenomena are associated with particle precipitation and field aligned currents (sub storms and auroras), and they are represented by the AE index; as well as by the equatorial ring current Dst index. It is an integral index of the magnetospheric activity (Hargreaves, 1992).
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The X-ray flux measured by GOES instruments corresponds to the solar background emission enhanced by several orders of magnitudes during solar flares. The electron flux is the particle population in the magnetosphere, which has a background component increased during flare and CME driven disturbances. Fig. 7 shows that the electron flux greater than 2 MeV at geosynchronous orbit reached high levels on November 03–04 (associated with the X8 flare and CME on November 02). These particles were injected during the magnetic storm on November 04, and decreased during the remaining of November 04, increasing again to high levels in November 05 due to effects of the X28 flare-CME of November 04. The Kp was low (around 3) until November 04 storm, when it reached a peak Kp 7 (value of a severe storm), recovering to quiet conditions during days 05 and most of November 06. After the second shock wave, at 1940 UT on November 06, magnetic activity increased to Kp 5 (moderate activity). X-ray flux bands present several peaks orders of magnitude above the background flux, which are associated with flare activity. A large number of peaks is present during November 3. During this day, 2X class, 1M class and 3C class flares were recorded. On November 04, the largest flare ever recorded happened, X28, and other 3M class and 3C class flares were observed. The X-ray flux decreases substantially on days 05 and 06, with peaks on day 05 caused by 2M class and 2C class flares. During this period, high level activity was observed on November 03 with an X2 flare at 01:30 UT and an X3 flare at 0955 UT, both from region 488. On November 04, the big X28 flare was observed, from region 486, beginning at 1930 UT and saturating GOES X-ray sensors at X17.4 level for 12 min. Further analyses
˚ ), and the Kp index for 3–6 November 2003. Fig. 7. GOES-12 measurements of electron flux (>2 MeV) and X-rays fluxes (0.5–4 and 1–8 A
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of the available data yielded an estimated X-ray peak flux of X28 at 1950 UT.
5. Other solar effects at earth natural systems 5.1. Aurora Auroras (northern or polar lights) are one of the most spectacular natureÕs phenomena. The electrons and ions energized by the solar wind energy transferred to the magnetosphere are guided by the geomagnetic field lines preferentially to the high latitudes – polar regions, where they can penetrate deep in the upper atmosphere and collide with local atoms/molecules; the energy exchanged in these collisions excite the atmospheric constituents to upper, excited energy levels; when they relax to their normal levels, they emit electromagnetic radiation mainly in visible and infrared regions. The maximum latitude of auroral occurrence is around 67 at midnight and 77 at noon, and the distribution zone of auroral frequency has an oval shape - the auroral oval. This auroral oval does not occur over the pole, but it is a ring circulating the polar regions. This is one of the most important boundaries of Geospace and it is generally considered to mark the division between the footpoints of open and closed field lines. When a storm or a sub-storm occurs, its boundaries are displaced to lower latitudes (Chamberlain, 1961; Akasofu, 1964; Akasofu and Chapman, 1972). 5.2. Solar proton events Energetic protons can reach Earth within 30 min or less after a major flareÕs peak. Some of these particles spiral down EarthÕs magnetic field lines, penetrating the upper layers of our atmosphere where they produce additional ionization and may produce a significant increase in the radiation environment. In addition, they can locally affect the chemistry of atmosphere, producing odd nitrogen compounds that react with atmospheric species and causes stratospheric ozone depletion, mainly in polar latitudes (Heath et al., 1997).
and it has been speculated that solar irradiance shall vary by a larger factor in longer periods – decades to centuries. A strong evidence for solar activity modulation of climate is the near coincidence of the weak solar activity during the Maunder minimum with a cold period during the Little Ice Age. Other possible postulated mechanism is the solar plasma variability affecting cosmic ray and these ones influencing the atmospheric electrical fields and cloud cover (Eddy, 1976; Hoyt and Schatten, 1997; Reid, 1997). Over short-term periods, studies have found correlations between solar flares, sector boundary crossings and solar energetic particles with atmospheric circulation, pressure, temperature and other variables. However, this kind of association still needs further confirmation of the correlations observed and their physical mechanism needs to be better established (Hoyt and Schatten, 1997). 5.4. Biology Indirect effects of solar activity on tree rings growth has been shown, most likely through local temperature or precipitation modulation (Hoyt and Schatten, 1997). There are also evidences that geomagnetic field variations can affect biological systems. A more accepted effect is the degradation of pigeonÕs navigational abilities during geomagnetic storms; pigeons and other migratory animals, such as dolphins and whales, have internal biological compasses composed of the mineral magnetite wrapped in bundles of nerve cells. Much more controversial is the suggestion from several studies that high solar/geomagnetic activity can increase the rate of heart and mental diseases in human beings, among other effects. The subject of cosmic electromagnetic fields effects on biological systems is still in its infancy and much further research is demanded.
6. Technology effects This summary on technological effects of space weather is mainly based in Tascione (1988) and Hargreaves (1992).
5.3. Climate 6.1. Ionospheric disturbance Solar radiative output is well known to drive climate and atmospheric characteristics, as it is shown by the diurnal and annual cycle of surface temperature. Whether solar active variations could affect the terrestrial weather and climate, however, has been a controversial topic over the last 150 years (Hoyt and Schatten, 1997). The finding that the so-called solar constant was actually varying with the sunspot cycle was possible only with satellite data in the last two decades. But the amount of variation is very small, around 0.1%,
6.1.1. Sudden ionospheric disturbances (SIDs) The SID manifests itself within a few minutes of the appearance of some strong solar flares as a sharp fadeout of long-distance radio-communications on the dayside of the Earth. The short-wave fadeout (SWF) is caused by strong enhancement of the electron density of the D and lower E-regions, presumably as a result of the penetration of solar X-rays to these low levels. High frequency radio waves, that normally would pass
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through the D-region and reflect at higher levels, are absorbed instead. The disruption of communications may last an-hour or so. 6.1.2. Polar cap absorption (PCA) PCA is observed only at high latitudes, where it is accompanied by communications blackout resulting from an increase in electron density at altitudes between 55 and 90 km. The effect is associated with the arrival of very energetic solar flare protons, which are guided by the EarthÕs magnetic field lines directly into the polar caps. There, they penetrate to altitudes as low as 50 km before giving up their energy in ionizing neutral atmospheric species. These protons may be delayed in arrival by their interaction with the magnetosphere. 6.2. Damages to space-based system 6.2.1. Spacecraft charging The spacecraft charging is a variation in the electrostatic potential of a spacecraft surface with respect to the surrounding plasma. During geomagnetic storms, the number and energy of electrons and ions increase. When a satellite travels through this energized environment, the charged particles striking it cause different portions of the spacecraft to be differentially charged. Eventually, electrical discharges can arc across spacecraft components, harming and possibly disabling them. Even weak discharges have been related to a variety of problems, including: spurious electronic switching activity; breakdown of vehicle thermal coatings; amplifier and solar cell degradations and degradation of optical sensors. 6.2.2. Single event upsets (SEUs) SEUs are bit flips in digital microelectronic circuits which can cause: damage to stored data; damage to software; make the central processing unit (CPU) to halt; the CPU to write over critical data tables; occurrence of various unplanned events including loss of mission. SEUs in space borne electronics are caused by the direct ionization of silicon material by a high energy ion passing through it. These high energy ions are heavy ion cosmic rays and also solar flare energetic particles, For satellites in near Earth orbits (less than four Earth radii) an extra factor is the particle population trapped in the radiation belts. 6.2.3. Single event latchups (SELs) SELs occur when digital microcircuits short their power supplies to ground. These events can cause inoperability; permanent failure of the affected components and computer failure. 6.2.4. Spacecraft drag Spacecrafts operating below a few thousand kilometers are subjected to effects of a significant number of
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atmospheric particles during each orbit. Changes in the atmospheric density at the vehicle altitude can rapidly and significantly change the vehicle orbit. Any mechanism capable of heating the EarthÕs atmosphere will produce density changes at altitudes above the heated level, such heating can be resulting from geomagnetic storms and changes in solar EUV radiation, for instance during a solar flare. 6.2.5. Space radiation The main hazard to life in space is found in the ionizing radiation resulting from exposure to high-energy particles. If a particle has sufficient kinetic energy it can pass through protective equipment and impact a crew memberÕs body. Natural radiation near Earth space has three primary components: galactic cosmic radiation,radiation produced by trapped particles in Van Allen belts and radiation from solar flare particles. Intense solar flares release very high-energy particles that can be as injurious to humans as the low-energy radiation from nuclear blasts. EarthÕs atmosphere and magnetosphere allow adequate protection for us on the ground, but astronauts in space are subject to potentially lethal dosages of radiation. The penetration of high-energy particles into living cells, measured as radiation dose, leads to chromosome damage and, potentially, cancer. Large doses can be fatal immediately. Solar protons with energies greater than 30 MeV are particularly hazardous. Solar proton events can also produce elevated radiation aboard aircraft flying at high altitudes. Although these risks are small, monitoring of solar proton events by satellite instrumentation allows the occasional exposure to be monitored and evaluated. High-energy particles degrade detector performance by the accumulation of material micro structural damage. Different devices have varying degrees of total dose vulnerability. 6.3. Damages to ground-based systems 6.3.1. Communication Many communication systems utilize the ionosphere to reflect radio signals over long distances. Ionospheric storms can affect radio communication at all latitudes. Some radio frequencies are absorbed and others are reflected, leading to rapidly fluctuating signals and unexpected propagation paths. TV and commercial radio stations are little affected by solar activity, but groundto-air, ship-to-shore, and amateur radio systems are frequently disrupted. 6.3.2. Navigation systems Systems such as LORAN and OMEGA, which were widely used until some years ago, were adversely affected when solar activity disrupts their signal propagation. Airplanes and ships used for several years the very low
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Table 1 Summary from space weather storm effects Flares
CMEs/flares
CMEs/coronal holes
Eletromagnetic radiation Arrival: 8 min Duration: 1–2 h X-rays, EUV, radio bursts Satellite/communications interference Radar interference Shortwave radio fades
High-energy particles Arrival: 15 min to few hours Duration: days Proton events Satellite disorientation False sensor readings Spacecraft damage Launch payload failure High altitude aircraft Radiation Shortwave radio fades
Low–medium energy particles Arrival: 2–4 days Duration: days Geomagnetic storms Spacecraft charging and drag Space track errors Launch trajectory errors Radar interference Radio propagation anomalies Power blackouts
frequency signals from these transmitters to determine their positions. During solar events and geomagnetic storms, the system can give navigators information that is inaccurate by as much as several kilometers. More modern GPS signals are also affected when solar activity causes sudden variations in the density of the ionosphere. 6.3.3. Electric power When magnetic fields move in the vicinity of a conductor such as a wire, an electric current is induced in the conductor. This happens in a large scale during geomagnetic storms. Power companies transmit energy to their customers via long transmission lines. The nearly direct currents induced in these lines from geomagnetic storms are harmful to electrical transmission equipment. On March 13, 1989, in Montreal, Quebec, 6 million people were without commercial electric power for 9 h as a result of a huge geomagnetic storm. Some areas in the northeastern US and in Sweden also experienced blackouts.
spheric main aspects of solar–terrestrial phenomena were approached through an example, the November 2003 event. Some technological effects were also summarized. A number of important questions remain to be answered in Space Weather studies: can we predict the arrival of a solar disturbance to Earth? Can we predict magnetic field, velocity and other characteristics of interplanetary disturbances? Can we predict the occurrence of flares and other high-energy particle events? Which would be the consequences of a super-storm on the present day technology? The answers for these and other questions will came from the basic research in all Space Weather aspects: solar, interplanetary, magnetospheric, ionospheric and atmospheric sciences and basic plasma physics and electrodynamics. Also of extreme importance will be the comparative studies between solar system planetary magnetospheres and atmospheres and in a not so far future, with extra-solar systems.
6.3.4. Pipelines Rapidly fluctuating geomagnetic fields can induce currents into pipelines. During these times, several problems can arise for pipeline engineers. Flow meters in the pipeline can transmit erroneous flow information, and the corrosion rate of the pipeline is dramatically increased. Table 1 summarizes the main space weather storm effects as related to its solar origins, the type of disturbance generated near Earth (electromagnetic and particle) and its impacts on technological systems. More details about space weather effects on technology systems can be found in the following references: Lanzerotti (1979), Garrett (1981), Jansen and Pirjola (2004) and references therein.
Acknowledgments
7. Summary
References
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The authors acknowledge FAPESP (02/12723-2 and 02/14150-0) for Post-doctoral fellowship and CAPES for Doctoral fellowship. Thanks to ACE/MAG and ACE/SWEPAM for magnetic field and interplanetary plasma data. Thanks to World Data Centre for Geomagnetism for Dst index data. Thanks to Space Physics Interactive Data Resource/NOAA for Kp index and GOES-12 X-ray and electron flux data. Thanks to SOHO/LASCO, SOHO/EIT AND SOHO/MDI work teams for kindly put images available on internet. SOHO is a project of international cooperation between ESA and NASA.
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Hoyt, D.V., Schatten, K.H. The Role of the Sun in Climate Change. Oxford University Press, Oxford, 1997. Hoyt, D.V., Schatten, K.H. Group sunspot numbers: a new solar activity reconstruction. Solar Phys. 179, 189–219, 1998. Hundhausen, A.J. Coronal Expansion and Solar Wind. Springer, Berlin, 1972. Jansen, F., Pirjola, R. Space weather research elucidates risks to technological infrastructure. EOS 85 (25), 241, 245–246, 2004. Kamide, Y., Baumjohann, W., Daglis, I.A., Gonzalez, W.D., Grande, M., Joselyn, J.A., McPherron, R.L., Phillips, J.L., Reeves, E.G.D., Rostoker, G., Sharma, A.S., Singer, H.J., Tsurutani, B.T., Vasyliunas, V.M. Current understanding of magnetic storms: storm– substorm relationships. J. Geophys. Res. 103, 17705–17728, 1998. Kivelson, M.G., Russell, C.T. Introduction to Space Physics. Cambridge University Press, New York, 1995. Lanzerotti, L.J. Impacts of ionospheric/magnetospheric process on terrestrial science and technology, in: Lanzerotti, L.J., Kennel, C.F., Parker, E.N. (Eds.), Solar System Plasma Physics, vol. III. North-Holland Publishing Company, New York, pp. 319–363, 1979. Menvielle, M., Berthelier, A. The K-derived planetary indices: description and availability. Rev. Geophys. 29, 415–432, 1991. Neugebauer, M., Snyder, C.W. Mariner 2 observations of the solar wind. 1-Average properties. J. Geophys. Res. 71, 4469–4475, 1966. Parker, E.N. Dynamics of the interplanetary gas and magnetic fields. Astrophys. J. 128, 664–676, 1958. Reid, G.C. Solar forcing of global climate change since the mid-17th century. Clim. Change 37, 391–405, 1997. Rostoker, G. Geomagnetic indices. Rev. Geophys. 10, 935–950, 1972. Russell, C.T. The configuration of the magnetosphere, 1–16, in: Dyer, E.R. (Ed.), Critical Problems of Magnetospheric Physics, IUCSTP, Natl. Acad. Sci, Washington, DC, 1972. Schatten, K.H. Large-scale properties of the interplanetary magnetic field. Rev. Geophys. Space Phys. 9, 773–792, 1971. Siscoe, G. The space-weather enterprise: past, present and future. J. Atmos. Solar–Terr. Phys. 62, 1223–1232, 2000. Schwenn, R. Relationship of coronal transients to interplanetary shocks – 3D aspecs. Space Sci. Rev. 44, 139–168, 1986. Sturrock, P.A. (Ed.). Solar Flares. Colorado Associated Universty Press, Boulder, CO, 1980. Svestka, Z. Solar Flares. D. Reidel, Dordrecht and Boston, 1976. Tascione, T.F. Introduction to the Space Environment (Orbit, a Foundation Series). Krieger Publishing Company, New York, 1988.
Introduction to space weather E. Echer *, W.D. Gonzalez, F.L. Guarnieri, A. Dal Lago, L.E.A. Vieira Instituto Nacional de Pesquisas Espaciais (INPE), Minsterio da ciencia e Technologia, Av.dos Astronautas, P.O. Box 515, Sa˜o Jose´ dos Campos 12245-970, SP, Brazil Received 1 June 2004; received in revised form 27 January 2005; accepted 22 February 2005
Abstract The solar and interplanetary origin of space weather disturbances, as well as the related magnetospheric dynamics, will be presented. Besides the involved phenomenology in solar–terrestrial physics, some of the main effects of space weather variability concerning mankind in space and at the earthÕs surface will also be discussed. The November 2003 event is shown as an example of the solar, interplanetary and magnetospheric aspects of a space weather storm. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Space weather; Solar effects; Magnetospheric dynamics; Solar–terrestrial physics
1. Introduction In the last decades many technological advances occurred and mankind has become more dependent on the space systems and satellite-based services, and from other ground-based technologies, which are influenced by Sun–Earth interaction phenomena. The region where the electromagnetic Sun–Earth interactions are more important is called the Geospace (Hargreaves, 1992) and it encompasses the EarthÕs upper atmosphere (neutral and ionized), the magnetosphere and the local solar wind medium. Both academic and practical interests in the study of this environment led to the creation in recent years of a new field, the Space Weather, which was formerly known as solar–terrestrial physics. The objective of space weather research is to understand in theory and practice the, stormy and hostile Geospace environment, in order to make possible procedures that turn it safe for human technological activities and for the human presence in space (Hargreaves, 1992; Baker, 2000; Siscoe, 2000; Cole, 2003). *
Corresponding author. Fax: +55 12 3945 6810. E-mail address: [email protected] (E. Echer).
This environment is a complex system with three parts: the Sun and its atmosphere as the origin of the energy, the interplanetary space as the propagation medium and the EarthÕs magnetosphere and upper atmosphere as the forced system where the energy originated at the Sun and propagated through the interplanetary space is deposited. The occurrence of solar activity manifestations propagating through the interplanetary space and interacting with the terrestrial magnetosphere origins phenomena known for centuries, such as auroras (Chamberlain, 1961; Akasofu, 1964) and geomagnetic field disturbances – the geomagnetic storms (Chapman and Ferraro, 1931). Geomagnetic storms are usually defined by the geomagnetic field horizontal component variations, but they actually are disturbances in the plasma populations present in the entire magnetosphere. Nowadays, it is well known that the prime cause of geomagnetic storms is the presence of a southward interplanetary magnetic field structure in the solar wind. This magnetic field orientation makes possible the energy to be transferred to the EarthÕs magnetosphere through reconnection mechanism (Dungey, 1961; Gonzalez et al., 1994, 1999; Kamide et al., 1998). Although geomagnetic storms have
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.02.098
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been studied for several decades, their solar and interplanetary origins have become more clear only with more recent observations, especially in the last 10 years, when a continuous monitoring of the solar activity and interplanetary propagation conditions has been done. Nevertheless, the solar-interplanetary space–magnetospheric–atmospheric chain is very complex and there are many points to be further studied and better understood. The aim of this short review is to give some basic information on this environment, showing an example of a space weather storm during the beginning of November 2003. A brief summary of space weather effects on technology systems is also presented.
2. Solar activity The energy driving the Geospace processes has its fundamental origin in the SunÕs core. Nuclear fusion inside the SunÕs core generates radiative energy that propagates outward, and subsequently is transformed in convective motion energy in sub-surface layers. The dynamo action produces intense magnetic fields in the solar atmosphere, which are transported outward to the interplanetary space with the plasma in the solar corona, constituting the solar wind (Parker, 1958; Neugebauer and Snyder, 1966; Hundhausen, 1972; Kivelson and Russell, 1995). The magnetized solar wind flows continuously by the interplanetary medium and interacts with planetary magnetic fields, delimiting their magnetospheres. Besides the presence of this background and continuous solar wind, the interplanetary space is permeated in solar activity transients, e.g., the remnants of solar flares and coronal mass ejections (CMEs). The easiest observable characteristic of the solar variability is the number of sunspots in the visible solar hemisphere. The records of the observed sunspot number show an average regular cycle of solar activity near 11 years (Eddy, 1976). Fig. 1 shows the annual average
Fig. 1. Wolf and group sunspot number annual averages 1610–2000.
Wolf sunspot number (RZ), which is available since 1700, and the group sunspot number (RG) which is available since 1610. It is easily seen the 11-year oscillation, as well as the long-term variations, with sunspot maxima alternatively higher and lower. Years 1800– 1830 are particularly low solar activity period, being known as the Dalton minimum. The period of 1645– 1715 had the lowest sunspot activity ever observed, being called Maunder minimum (Eddy, 1976; Hoyt and Schatten, 1998). Solar transient events occur sporadically, but its frequency is higher around sunspot maximum. The sunspot number is considered then a general indicator of the solar magnetic activity. Nevertheless, the RZ is known to be less accurate before 1850. The group sunspot number RG was idealized to be a better descriptor of the solar activity, being available since 1610 (Hoyt and Schatten, 1998). Both RZ and RG data are available through Internet, from the National Geophysical Data Center, Boulder, Colorado (http://www.ngdc.noaa.gov/). Sunspots are solar disk regions that are darker than the vicinity areas and contain strong and transient magnetic fields. They are formed and dissipated over periods of days to weeks, more rarely persisting for a few solar rotations. They occur when strong magnetic fields emerge through the solar surface, partially blocking the plasma convection from the bottom and allowing the sunspot area to be cooler than the photosphere, around 4200 C against 6000 C of solar background. This difference in temperature is the reason why this area appears as a dark spot on the solar surface. The darkest area at the center of a sunspot, called umbra, is the region where the magnetic field strengths are the highest. The less dark, striated area around the umbra is called penumbra. Sunspots co rotate with the solar surface, taking about 27 days to make a complete rotation as seen from Earth. Sunspots near the SunÕs equator rotate at a faster rate than those near the solar poles. Groups of sunspots, especially those with complex magnetic field configurations, are often the sites of flares (Svestka, 1976; Sturrock, 1980). Fig. 2 shows an example of sunspots seen on solar disk through the SOHO Michelson Doppler Imager ˚ (http:// (MDI) instrument that operates at 6767 A umbra.nascom.nasa.gov/images/latest_mdi_igram.gif). This image is for the November 03, 2003 event. The October–November 2003 months were a period of intense solar activity. Giant sunspot regions 486 and 488 were seen near the solar limb. The solar magnetic field extends through solar atmosphere to the interplanetary space. The coronal gas is ionized and it is an excellent electrical conductor, despite its low density. Thus this gas is free to move parallel to magnetic field lines, but not perpendicularly. The solar corona structure is complex, but it is basically constituted of two regions characterized by open and closed
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˚. Fig. 2. Solar disk observed by SOHO MDI instrument at 6767 A Sunspots are seen as dark regions on the disk. Daily image for November 03, 2003.
magnetic field lines, shown schematically in Fig. 3. In space physics, open magnetic field lines are the ones that stretch themselves to long distances in the interplanetary space, reconnecting at long distances in the heliosphere. Open field lines occur in coronal holes, which emit the high speed solar wind streams. A closed magnetic field line is anchored in the photosphere in two points, being stretched in the corona as a loop. Low speed solar wind streams are originated in the helmet streamer and heliospheric current sheet. The helmet streamer is located, during solar minimum, close to the solar equatorial plane, while during solar maximum it is found over a large latitudinal range (Schatten, 1971).
Fig. 3. Simplified scheme of the solar corona magnetic field topology – open (coronal holes) and closed (helmet streamer) magnetic field lines are drawn.
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Large disturbances in the space weather, such as intense geomagnetic storms, shock waves and energetic particle events are mostly associated with two solar activity transient phenomena: solar flares and coronal mass ejections (CMEs). These two events seem to be part of a single phenomenon, a solar magnetic eruption. Nowadays it seems that neither one is the cause of the other (Gosling, 1993). Solar flares are intense, temporary releases of energy that were first observed by Carrington in 1859. sunspot. The primary energy source for flares appears to be the tearing and reconnection of strong magnetic fields. Flares radiate throughout the electromagnetic spectrum, from gamma rays and X-rays, through visible light out to kilometric radio waves. Flares are important to space weather mainly because they appear in connection to some CMEs and also because they have an important role in particle acceleration. The enhanced X-ray and extreme ultraviolet (EUV) solar radiation during a flare causes a dramatic increase in ionospheric ionization, with several consequences for radio-propagation and telecommunication systems (Svestka, 1976; Sturrock, 1980). Solar flares are classified according to its X-ray emis˚ in emission classes B (with sion in the band 1–8 A 6 2 peak < 10 W m ), C (peak between 10 6 and 10 5 W m 2), M (with peak between 10 5 and 10 4 Wm 2) and X (with peak > 10 4 Wm 2). A number is also indicated after the letter which gives the intensity of emission, each category having nine subdivisions, e.g., C1–C9, C9 equivalent to a peak emission of 9 · 10 5 W m 2. Events of the X type are big events that can cause planetary radio blackouts and long lasting radiation storms. Events of the M type are of medium size, which can cause minor radiation storms and brief radio-blackouts, mainly in the polar regions. Events of C and B types have very few effects on Earth. The outer solar atmosphere, the corona, is permeated by strong magnetic fields (Fig. 3). In the closed field regions, often above sunspot groups, the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas and magnetic fields called coronal mass ejections. A large CME can contain 1016 g of matter that can be accelerated to several thousands of km per seconds. CMEs usually travel at speeds between 500–1500 km/s and take 2 or 4 days to cross the 150 · 106 km that separates the Sun from the Earth. Less frequent, very fast CMEs can reach earth in one day or less. These very fast CMEs are often associated with flares, while the slow ones not. CMEs are monitored using coronagraphs, which produce artificial eclipses on the Sun by placing an Ôocculting diskÕ over the image of the Sun. CMEs directed along the Sun–Earth line can be detected as ‘‘halos’’ around the occulting disk in a white-light coronagraph. A CME is a full-halo when it extends 360 around the Sun and a partial halo
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Fig. 4. SOHO observations of 04 November 2003 event – solar flare (EIT) LASCO C2 (CME).
if the apparent width is greater than 120 and if it appears in projection above at least one of the poles; this latter criterion is an attempt to exclude limb events that just happen to be large but do not have a component along the radial Sun–Earth line. Limb CMEs occur near solar limb and are not aimed to the Earth (Gosling, 1997). Fig. 4 shows an example of a solar flare and a CME occurred on November 04, 2003. This flare is considered the biggest ever recorded in X-rays measurements by GOES, classified as of X28 type flare. The image is from SOHO Extreme ultraviolet Imaging Telescope (EIT) Fe ˚ line. The associated CME is shown in Fig. XII 195 A 4(b), as seen from SOHO Large Angle and Spectrometric Coronagraph (LASCO) C2 in white light. Both events were associated with the big sunspot groups seen in Fig. 2. The CME in Fig. 4 is a halo CME.
3. Interplanetary propagation The region between the Sun and the planets has been named the interplanetary medium. Although in the past it was considered a perfect vacuum, since the beginning of the 1960s it is known to be a turbulent region dominated by the solar wind, which flows at velocities in the range of 250–1000 km/s. Other characteristics of the solar wind, such as density, composition, and magnetic field strength, among others, vary with changing conditions on the Sun. The effect of the solar wind can be seen in the tail of comets, part of which always points away from the Sun. Near Earth, the solar wind is basically a proton–electron gas that flows from the sun (and outward to the entire solar system), with a velocity typically around 400–500 km/s, density around 5 cm 3 and interplanetary magnetic field strength of 5 nT. It is the solar atmospheric gas escaping as a result of the high temper-
ature corona expansion. Solar wind expansion is supersonic, which means that it propagates with higher velocity than the characteristic velocity of the medium – the magnetosonic speed (Hundhausen, 1972). Since the solar wind is high conductive plasma, the solar magnetic field is frozen in it and magnetic field lines are dragged by the solar plasma flow. As the Sun rotates, the solar magnetic field is deformed in a spiral form – the Parker spiral – whose angle is around 45 from the Sun–Earth line near 1 AU (1 AU = average Sun–Earth distance, 150 · 106 km). The solar wind flow deforms magnetic obstacles, such as planets and their satellites, but the resulting structure is dependent on their magnetic fields and atmospheres. Considering the Earth, its dipolar magnetic pattern field is deformed, compressed in the sunward direction and stretched out in the anti-solar direction, forming a long magnetotail. This interaction creates the magnetosphere as a complex magnetic cavity around Earth (Russell, 1972; Kivelson and Russell, 1995). The interplanetary counterparts of coronal mass ejections (ICMES) at the Sun have been identified since the early years of solar wind observations. A sub-set of ICMEs are the magnetic clouds (MC), which seem to constitute around 1/3 of all the ICMES (Gosling, 1997). These structures are identified, near 1 AU, by their high magnetic field intensity, low proton temperature, low Beta (ratio between thermal and magnetic pressure), and a smooth and large scale rotation in one of the magnetic field components. ICMEs present dimensions around 0.2–0.3 AU and cross the spacecrafts or Earth in 24 h (Burlaga et al., 1981). Current models of these magnetic clouds consider them as giant magnetic flux ropes with field aligned currents. Other ICMEs are believed to be ‘‘complex ejecta’’, with a disordered magnetic field (Burlaga et al., 2001). Presently it is unknown whether all ICMEs are flux ropes or not.
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It has been suggested that they all could be magnetic clouds, that are observed as complex structure due to solar wind interactions (e.g., with co-rotating streams or other ICMEs), or due to the spacecraft measuring its parameters far from the ICME central axis. On the other hand, one can not discard the possibility of different magnetic field topologies be expelled from the Sun. Plasma and magnetic field (5 min resolution observations) observed by instruments on board of the Advanced Composition Explorer (ACE) spacecraft are shown in Fig. 5. Panels are, from top to bottom, proton temperature Tp (in 105 K), solar wind speed Vsw (in km/s), solar wind proton density Np (in cm 3), magnetic field strength B (nT), magnetic field BX, BY and BZ components in GSE (Geocentric Solar–Ecliptic coordinates) and the plasma b parameter (b = pk/pB, kNpTp/(B2/l0)) Two shock waves are indicated by the continuous lines. A southward intense structure is indicated with Bs and gray color. Collisionless shock waves are disturbances propagating in the solar wind with relative velocity (to solar wind) higher than the characteristic velocity, and are usually driven by the interplanetary remnants of CMEs–ICMEs. They are seen in the interplanetary data as an abrupt increase in plasma and magnetic field parameters. In the present example, it was not possible to find the signals of an ICME, which means that it probably missed the Earth. The shock was observed be-
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cause it has a larger extent in area than the driving interplanetary structures (Schwenn, 1986). The first shock was driven by the CME associated with the X8 flare on November 02, 2003, and passed by ACE spacecraft at 0600 UT on November, 04, 2003, with Vsw increasing from 500 to 800 km/s. A turbulent BZ variation is present, reaching 20 nT during a few hours, then returning to north conditions and to 0 values after. Solar wind speed gradually declined to 500 km/s on November 06, until a second CME shock arrives. This was driven by the CME associated with the X28 flare on November 04 (Fig. 4) and that arrived at ACE on 06 November with Vsw reaching 625 km/s. Large solar proton events contaminated the plasma instrument onboard ACE, making data unreliable during November 02–03. The BS structure shown in this Figure was responsible for a geomagnetic storm that followed it on November 04. The basic nature of geomagnetic field-solar wind interactions was first showed by Chapman and Ferraro (1931). This interaction is based on two theoretical principles: Plasma and magnetic fields behaves approximately as if they were frozen in to one another. This is a consequence of the FaradayÕs law, because in electrically conductive plasma, the electric field in the rest frame should be close to zero, or very large electrical currents will be induced. Magnetic field lines are then
Fig. 5. Interplanetary plasma and magnetic field parameters observed by ACE spacecraft during the November 3–6, 2003 period. Two interplanetary shocks are marked by the continuous vertical lines.
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transported by plasmas and they are warped and twisted as the flux moves. Example: the IMF is twisted in a large spiral structure due to solar rotation. Effect of magnetic field on the plasma: it comes from the Lorentz force, experimented by a charge q moving with velocity in a magnetic field. Summing up on all charges in a given region, the net force generally is opposite to the inclination and twist of the magnetic field, or to its compression, in the frozen flux. There are two components of this force: (a) The magnetic field does an effective pressure on the plasma proportional to the square magnetic field magnitude. This force opposes to compression/rarefactions in the magnetic field; (b) The warped/twisted magnetic field lines do stress forces over the plasma. These forces oppose to the warping and twisting in magnetic field lines. As the solar wind plasma is frozen in the interplanetary magnetic field, and the terrestrial magnetosphere plasma is frozen in the geomagnetic field, the two plasma populations do not mix. The magnetosphere is a ÔclosedÕ system when the IMF BZ is northward, but it becomes an open system when BZ is southward, due to mass–energy transfer through reconnection mechanism (Dungey, 1961). The general evidence is that the dawndusk solar wind electric field drives the magnetospheric convection. These electric fields are caused by a combination of solar wind velocity and to the north–south interplanetary magnetic field. From these two parameters, the IMF north–south component is the most important because of its largest variability (Gonzalez et al., 1994, 1999).
4. Magnetospheric impacts Around the middle of the XIX century, the disturbances that showed a large decrease in the horizontal geomagnetic field component in low latitudes were denominated magnetic storms (Chapman and Bartels, 1940; Kamide et al., 1998). The characteristic signal of a magnetic storm is the horizontal (H) component depression of the geomagnetic field during several hours. This depression is caused by the intensification of the ring current circulating Earth in the westward direction and it is monitored by the Dst index. A magnetic storm sometimes presents a sudden increase in the H component of the geomagnetic field (storm sudden commencement) followed by an arbitrary period during which the enhanced field do not change substantially (initial phase). This is followed by a reduction in the H component during some hours in the main phase and ending with a slow recovery phase, during tens of hours. The initial phase and the sudden impulse are not always
present in magnetic storms, which are characterized essentially by the presence of the main and recovery phase (Gonzalez et al., 1994; Kamide et al., 1998). Fig. 6 shows an example of the Dst index variation during a magnetic storm, for the November 3–6, 2003 period. The storm presents all the characteristics of a typical storm during solar maximum conditions, with an initial phase and sudden impulse (due to the first interplanetary shock showed in Fig. 5), followed by a main phase, which was caused by the sheath fluctuating field after this shock. A large sudden impulse, 30 nT, is seen due to the strong magnetospheric compression by the shock. The main phase is very fast, characteristic of BS sheath caused storms. This storm reached a minimum of 90 nT, being a moderate storm, not reaching the intense storm criteria, Dst < 100 nT (Gonzalez et al., 1994, 1999). The recovery phase occurred during November 5 and 6, being terminated by the second sudden impulse due to the arrival of the second shock wave. The Dst index is based on the hourly average variations of the H component recorded at four low-latitudes observatories, after subtracting the average solar quiet variation and the permanent magnetic field from the disturbed one. It is available since 1957. The depression in the H component of the geomagnetic field is caused by an enhancement in the ring current, which is constituted primarily by energetic ions that flow in the westward direction in the region of 4–6 terrestrial radii, during the growth of the storm main phase. The recovery phase is characterized by a decay of the ring current due to a combination of several different energetic particle losses. The processes mostly considered for the decay of the ring current are: charge exchange, Coulomb collisions, wave-particle interactions, outflow of ring current ions through the dayside magnetopause. The Dst index is thus a measure of the kinetic energy carried by the particles in the ring current. Ideally the Dst index should be a measure of the symmetric ring current strength, but
Fig. 6. Geomagnetic storm of 4 November 2003. The storm phases are marked.
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the H component measured at any station contains effects of many other current systems, such as the magnetopause current, partial ring current and tail current, among others (Akasofu, 1983; Kivelson and Russell, 1995). Fig. 7 shows other parameters measured near Earth: 5-min averaged integral electron flux (electrons cm 2 s 1 sr 1) with energies greater man 2 MeV observed at GOES-12 geostationary satellite; 5-min averaged X-ray flux (Wm 2) measured by GOES-12 in ˚ band; the 3-h Kp index. 0.5–4 and in 1–8 A The 3-h Kp index represents the intensity of the planetary magnetic activity as seen at sub-auroral latitudes. The K index, derived for each of the contributing midlatitude observatories, reflects the maximum range of a component of the field over the 3-h time interval at each station. The Kp index is the average of the K values from all contributing observatories. A conversion scale transforms the quasi-logarithm Kp to a linear index named ap. This index is available since 1932 (Rostoker, 1972; Menvielle and Berthelier, 1991). Other index widely used is the auroral eletrojet (AE) index, which is idealized to obtain a measure of the strength of the auroral eletrojets. It is calculated as the difference between the upper (AU index) and lower (AL index) envelopes of the magnetic field H-component disturbances in the auroral zone (Rostoker, 1972). The Kp/ap index is, then, a mid-latitude index that is sensitive to both auroral phenomena and to the equatorial ring current index. The auroral phenomena are associated with particle precipitation and field aligned currents (sub storms and auroras), and they are represented by the AE index; as well as by the equatorial ring current Dst index. It is an integral index of the magnetospheric activity (Hargreaves, 1992).
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The X-ray flux measured by GOES instruments corresponds to the solar background emission enhanced by several orders of magnitudes during solar flares. The electron flux is the particle population in the magnetosphere, which has a background component increased during flare and CME driven disturbances. Fig. 7 shows that the electron flux greater than 2 MeV at geosynchronous orbit reached high levels on November 03–04 (associated with the X8 flare and CME on November 02). These particles were injected during the magnetic storm on November 04, and decreased during the remaining of November 04, increasing again to high levels in November 05 due to effects of the X28 flare-CME of November 04. The Kp was low (around 3) until November 04 storm, when it reached a peak Kp 7 (value of a severe storm), recovering to quiet conditions during days 05 and most of November 06. After the second shock wave, at 1940 UT on November 06, magnetic activity increased to Kp 5 (moderate activity). X-ray flux bands present several peaks orders of magnitude above the background flux, which are associated with flare activity. A large number of peaks is present during November 3. During this day, 2X class, 1M class and 3C class flares were recorded. On November 04, the largest flare ever recorded happened, X28, and other 3M class and 3C class flares were observed. The X-ray flux decreases substantially on days 05 and 06, with peaks on day 05 caused by 2M class and 2C class flares. During this period, high level activity was observed on November 03 with an X2 flare at 01:30 UT and an X3 flare at 0955 UT, both from region 488. On November 04, the big X28 flare was observed, from region 486, beginning at 1930 UT and saturating GOES X-ray sensors at X17.4 level for 12 min. Further analyses
˚ ), and the Kp index for 3–6 November 2003. Fig. 7. GOES-12 measurements of electron flux (>2 MeV) and X-rays fluxes (0.5–4 and 1–8 A
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of the available data yielded an estimated X-ray peak flux of X28 at 1950 UT.
5. Other solar effects at earth natural systems 5.1. Aurora Auroras (northern or polar lights) are one of the most spectacular natureÕs phenomena. The electrons and ions energized by the solar wind energy transferred to the magnetosphere are guided by the geomagnetic field lines preferentially to the high latitudes – polar regions, where they can penetrate deep in the upper atmosphere and collide with local atoms/molecules; the energy exchanged in these collisions excite the atmospheric constituents to upper, excited energy levels; when they relax to their normal levels, they emit electromagnetic radiation mainly in visible and infrared regions. The maximum latitude of auroral occurrence is around 67 at midnight and 77 at noon, and the distribution zone of auroral frequency has an oval shape - the auroral oval. This auroral oval does not occur over the pole, but it is a ring circulating the polar regions. This is one of the most important boundaries of Geospace and it is generally considered to mark the division between the footpoints of open and closed field lines. When a storm or a sub-storm occurs, its boundaries are displaced to lower latitudes (Chamberlain, 1961; Akasofu, 1964; Akasofu and Chapman, 1972). 5.2. Solar proton events Energetic protons can reach Earth within 30 min or less after a major flareÕs peak. Some of these particles spiral down EarthÕs magnetic field lines, penetrating the upper layers of our atmosphere where they produce additional ionization and may produce a significant increase in the radiation environment. In addition, they can locally affect the chemistry of atmosphere, producing odd nitrogen compounds that react with atmospheric species and causes stratospheric ozone depletion, mainly in polar latitudes (Heath et al., 1997).
and it has been speculated that solar irradiance shall vary by a larger factor in longer periods – decades to centuries. A strong evidence for solar activity modulation of climate is the near coincidence of the weak solar activity during the Maunder minimum with a cold period during the Little Ice Age. Other possible postulated mechanism is the solar plasma variability affecting cosmic ray and these ones influencing the atmospheric electrical fields and cloud cover (Eddy, 1976; Hoyt and Schatten, 1997; Reid, 1997). Over short-term periods, studies have found correlations between solar flares, sector boundary crossings and solar energetic particles with atmospheric circulation, pressure, temperature and other variables. However, this kind of association still needs further confirmation of the correlations observed and their physical mechanism needs to be better established (Hoyt and Schatten, 1997). 5.4. Biology Indirect effects of solar activity on tree rings growth has been shown, most likely through local temperature or precipitation modulation (Hoyt and Schatten, 1997). There are also evidences that geomagnetic field variations can affect biological systems. A more accepted effect is the degradation of pigeonÕs navigational abilities during geomagnetic storms; pigeons and other migratory animals, such as dolphins and whales, have internal biological compasses composed of the mineral magnetite wrapped in bundles of nerve cells. Much more controversial is the suggestion from several studies that high solar/geomagnetic activity can increase the rate of heart and mental diseases in human beings, among other effects. The subject of cosmic electromagnetic fields effects on biological systems is still in its infancy and much further research is demanded.
6. Technology effects This summary on technological effects of space weather is mainly based in Tascione (1988) and Hargreaves (1992).
5.3. Climate 6.1. Ionospheric disturbance Solar radiative output is well known to drive climate and atmospheric characteristics, as it is shown by the diurnal and annual cycle of surface temperature. Whether solar active variations could affect the terrestrial weather and climate, however, has been a controversial topic over the last 150 years (Hoyt and Schatten, 1997). The finding that the so-called solar constant was actually varying with the sunspot cycle was possible only with satellite data in the last two decades. But the amount of variation is very small, around 0.1%,
6.1.1. Sudden ionospheric disturbances (SIDs) The SID manifests itself within a few minutes of the appearance of some strong solar flares as a sharp fadeout of long-distance radio-communications on the dayside of the Earth. The short-wave fadeout (SWF) is caused by strong enhancement of the electron density of the D and lower E-regions, presumably as a result of the penetration of solar X-rays to these low levels. High frequency radio waves, that normally would pass
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through the D-region and reflect at higher levels, are absorbed instead. The disruption of communications may last an-hour or so. 6.1.2. Polar cap absorption (PCA) PCA is observed only at high latitudes, where it is accompanied by communications blackout resulting from an increase in electron density at altitudes between 55 and 90 km. The effect is associated with the arrival of very energetic solar flare protons, which are guided by the EarthÕs magnetic field lines directly into the polar caps. There, they penetrate to altitudes as low as 50 km before giving up their energy in ionizing neutral atmospheric species. These protons may be delayed in arrival by their interaction with the magnetosphere. 6.2. Damages to space-based system 6.2.1. Spacecraft charging The spacecraft charging is a variation in the electrostatic potential of a spacecraft surface with respect to the surrounding plasma. During geomagnetic storms, the number and energy of electrons and ions increase. When a satellite travels through this energized environment, the charged particles striking it cause different portions of the spacecraft to be differentially charged. Eventually, electrical discharges can arc across spacecraft components, harming and possibly disabling them. Even weak discharges have been related to a variety of problems, including: spurious electronic switching activity; breakdown of vehicle thermal coatings; amplifier and solar cell degradations and degradation of optical sensors. 6.2.2. Single event upsets (SEUs) SEUs are bit flips in digital microelectronic circuits which can cause: damage to stored data; damage to software; make the central processing unit (CPU) to halt; the CPU to write over critical data tables; occurrence of various unplanned events including loss of mission. SEUs in space borne electronics are caused by the direct ionization of silicon material by a high energy ion passing through it. These high energy ions are heavy ion cosmic rays and also solar flare energetic particles, For satellites in near Earth orbits (less than four Earth radii) an extra factor is the particle population trapped in the radiation belts. 6.2.3. Single event latchups (SELs) SELs occur when digital microcircuits short their power supplies to ground. These events can cause inoperability; permanent failure of the affected components and computer failure. 6.2.4. Spacecraft drag Spacecrafts operating below a few thousand kilometers are subjected to effects of a significant number of
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atmospheric particles during each orbit. Changes in the atmospheric density at the vehicle altitude can rapidly and significantly change the vehicle orbit. Any mechanism capable of heating the EarthÕs atmosphere will produce density changes at altitudes above the heated level, such heating can be resulting from geomagnetic storms and changes in solar EUV radiation, for instance during a solar flare. 6.2.5. Space radiation The main hazard to life in space is found in the ionizing radiation resulting from exposure to high-energy particles. If a particle has sufficient kinetic energy it can pass through protective equipment and impact a crew memberÕs body. Natural radiation near Earth space has three primary components: galactic cosmic radiation,radiation produced by trapped particles in Van Allen belts and radiation from solar flare particles. Intense solar flares release very high-energy particles that can be as injurious to humans as the low-energy radiation from nuclear blasts. EarthÕs atmosphere and magnetosphere allow adequate protection for us on the ground, but astronauts in space are subject to potentially lethal dosages of radiation. The penetration of high-energy particles into living cells, measured as radiation dose, leads to chromosome damage and, potentially, cancer. Large doses can be fatal immediately. Solar protons with energies greater than 30 MeV are particularly hazardous. Solar proton events can also produce elevated radiation aboard aircraft flying at high altitudes. Although these risks are small, monitoring of solar proton events by satellite instrumentation allows the occasional exposure to be monitored and evaluated. High-energy particles degrade detector performance by the accumulation of material micro structural damage. Different devices have varying degrees of total dose vulnerability. 6.3. Damages to ground-based systems 6.3.1. Communication Many communication systems utilize the ionosphere to reflect radio signals over long distances. Ionospheric storms can affect radio communication at all latitudes. Some radio frequencies are absorbed and others are reflected, leading to rapidly fluctuating signals and unexpected propagation paths. TV and commercial radio stations are little affected by solar activity, but groundto-air, ship-to-shore, and amateur radio systems are frequently disrupted. 6.3.2. Navigation systems Systems such as LORAN and OMEGA, which were widely used until some years ago, were adversely affected when solar activity disrupts their signal propagation. Airplanes and ships used for several years the very low
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Table 1 Summary from space weather storm effects Flares
CMEs/flares
CMEs/coronal holes
Eletromagnetic radiation Arrival: 8 min Duration: 1–2 h X-rays, EUV, radio bursts Satellite/communications interference Radar interference Shortwave radio fades
High-energy particles Arrival: 15 min to few hours Duration: days Proton events Satellite disorientation False sensor readings Spacecraft damage Launch payload failure High altitude aircraft Radiation Shortwave radio fades
Low–medium energy particles Arrival: 2–4 days Duration: days Geomagnetic storms Spacecraft charging and drag Space track errors Launch trajectory errors Radar interference Radio propagation anomalies Power blackouts
frequency signals from these transmitters to determine their positions. During solar events and geomagnetic storms, the system can give navigators information that is inaccurate by as much as several kilometers. More modern GPS signals are also affected when solar activity causes sudden variations in the density of the ionosphere. 6.3.3. Electric power When magnetic fields move in the vicinity of a conductor such as a wire, an electric current is induced in the conductor. This happens in a large scale during geomagnetic storms. Power companies transmit energy to their customers via long transmission lines. The nearly direct currents induced in these lines from geomagnetic storms are harmful to electrical transmission equipment. On March 13, 1989, in Montreal, Quebec, 6 million people were without commercial electric power for 9 h as a result of a huge geomagnetic storm. Some areas in the northeastern US and in Sweden also experienced blackouts.
spheric main aspects of solar–terrestrial phenomena were approached through an example, the November 2003 event. Some technological effects were also summarized. A number of important questions remain to be answered in Space Weather studies: can we predict the arrival of a solar disturbance to Earth? Can we predict magnetic field, velocity and other characteristics of interplanetary disturbances? Can we predict the occurrence of flares and other high-energy particle events? Which would be the consequences of a super-storm on the present day technology? The answers for these and other questions will came from the basic research in all Space Weather aspects: solar, interplanetary, magnetospheric, ionospheric and atmospheric sciences and basic plasma physics and electrodynamics. Also of extreme importance will be the comparative studies between solar system planetary magnetospheres and atmospheres and in a not so far future, with extra-solar systems.
6.3.4. Pipelines Rapidly fluctuating geomagnetic fields can induce currents into pipelines. During these times, several problems can arise for pipeline engineers. Flow meters in the pipeline can transmit erroneous flow information, and the corrosion rate of the pipeline is dramatically increased. Table 1 summarizes the main space weather storm effects as related to its solar origins, the type of disturbance generated near Earth (electromagnetic and particle) and its impacts on technological systems. More details about space weather effects on technology systems can be found in the following references: Lanzerotti (1979), Garrett (1981), Jansen and Pirjola (2004) and references therein.
Acknowledgments
7. Summary
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The authors acknowledge FAPESP (02/12723-2 and 02/14150-0) for Post-doctoral fellowship and CAPES for Doctoral fellowship. Thanks to ACE/MAG and ACE/SWEPAM for magnetic field and interplanetary plasma data. Thanks to World Data Centre for Geomagnetism for Dst index data. Thanks to Space Physics Interactive Data Resource/NOAA for Kp index and GOES-12 X-ray and electron flux data. Thanks to SOHO/LASCO, SOHO/EIT AND SOHO/MDI work teams for kindly put images available on internet. SOHO is a project of international cooperation between ESA and NASA.
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